Polymers are used in a wide variety of ways to enhance the production of oil or gas from underground formations. Usually the function of the polymer is to control the viscosity of the aqueous fluids which are injected into the formation. For example, in water flooding the efficiency of the water flood is improved by adding a water soluble polymer to the aqueous phase and thereby decreasing the mobility difference between the injected water and the oil in place. Polymers are also used in acidizing and/or fracture acidizing in which acidic compositions are used to stimulate production of hydrocarbon from underground formations by increasing the formation porosity. A water soluble or water dispersible polymer is incorporated to increase the viscosity of the fluid so that wider fractures can be developed and live acid can be forced farther into the formations. This increases the propant carrying capacity of the acid solutions and permits better fluid loss control.
Generally high molecular weight polymers (those having a molecular weight on average of at least 10.sup.6) or polymers with various gelling or crosslinking agents are used for this purpose. Most commercially available polymeric viscosifiers, however, are degraded by the hostile reservoir environment including high temperatures, acidity and extreme shear conditions, as well as by the electrolytes which are encountered in the oil recovery process. For example, hydrolyzed polyacrylamides fail in sea water solution at elevated temperatures due to precipitation of the polymer in the presence of calcium ions in the sea water. Xanthan polymers are insensitive to calcium ions but these polymers degrade at high temperatures and lose their viscosifying efficiency.
U.S. Pat. No. 4,579,667, Echt, et al. (1986) discloses gelled aqueous compositions containing an anionic partially hydrolyzed acrylamide and a water-soluble cationic polyamide-epihalohydrin resin useful in fracturing oil and gas formations in enhanced oil recovery. The invention is said to eliminate the need to use polyvalent metal salts and complexes in order to convert a water-soluble polymer to a crosslinked gel. Gels can be used to suspend propping agents, to reduce water loss by serving as a plugging agent, and as a "pusher" in surfactant flooding.
U.S. Pat. No. 4,690,219, Burns, et al. (1987) discloses acidizing well stimulation for oil production using an acid solution thickened with a copolymer of an N-vinyl lactam, such as N-vinyl-2-pyrrolidone, and an .alpha., .beta.-unsaturated amide, such as acrylamide. The thickening polymer operates in the absence of a crosslinking agent, and is said to function in hostile formation environments including temperatures above 170.degree. F. Aldehydes are disclosed, however, as suitable crosslinking agents to form a gel.
It is known that crosslinked polyvinylamines can be prepared in various ways. For example, Japanese Patent Publication No. J61051007-A (1986) discloses making a crosslinked poly(vinylamine) by copolymerizing N-vinylformamide with a copolymerizable crosslinkable monomer and then hydrolyzing the amide groups to amine. Japanese Patent Publication No. J61051006-A (1986) discloses, on the other hand, suspending a polyvinylamine in a dispersion medium and then crosslinking the polymer by reacting it with a dialdehyde or epichlorohydrin.
The preparation of polyvinylamines by hydrolysis of N-vinylformamide polymers is likewise well known. U.S. Pat. No. 4,623,699, Brunnmueller, et al. (1986) discloses making poly(vinylamines) by eliminating formyl groups from a polymer formed from N-vinylformamide using gaseous hydrogen chloride in the presence of not over 5 wt. % water, based on the polymer.
This application is related to U.S. Pat. No. 4,843,118 of Lai and Vijayendran which issued June 27, 1989, based on application Ser. No. 64,962 filed June 19, 1987. This patent discloses an acidized fracturing fluid containing poly(vinylamine) obtained by at least 50% hydrolysis of a poly(vinylamide) having a molecular weight of at least 10.sup.6. Background patents are referenced as disclosing methods for making poly(vinylamines) having lower molecular weights, normally involving solution polymerization. Crosslinking agents are optional and include organic titanate complexes, epichlorohydrin, hexamethylene diisocyanate, glyoxal, butanediol diacrylate, terephthaldehyde and glutaraldehyde. The disclosure of this patent, which is incorporated herein by reference, was filed as a continuation-in-part of Ser. No. 914,046, filed Oct. 1, 1986. This parent application and a related European Patent Application 0,264,649, published Apr. 24, 1988, disclose the preparation of poly(vinylamines) having at least 10.sup.6 average molecular weight by using either acid or base hydrolysis of poly(N-vinylformamide). The polymerization technique used is inverse emulsion polymerization. Among the utilities disclosed for such high molecular weight amines are uses in drilling mud compositions, cements for drilling holes, completion fluids, acidized fracturing fluids and general use in enhanced oil recovery. The parent application and the European patent application do not, however contain the disclosure found in the '118 patent concerning the use of crosslinking agents which can be incorporated into acidized fracturing fluids for well stimulation when such fluids contain high molecular weight poly(vinylamine). Application Ser. No. 914,046 is abandoned but the corresponding disclosure was filed in foreign countries and issued as Taiwan Patent No. 31922 on Aug. 10, 1989.
It is highly desirable to develop polymer gels which are stable and can be used as plugging agents in enhanced oil recovery. Such polymer gels are placed in high permeability zones in a formation so that subsequently injected treatment fluid is forced into the low permeability zones thereby giving better sweep efficiency by increasing the contact of the flooding fluid with the oil bearing reservoir sands.